Powder evaporation apparatus



Oct. 3, 1967 E. v. JENNINGS POWDER EVAPORATION APPARATUS Filed Aug. 30, 1965 2 Sheets-Sheet l INVENTOR. [Ema/W KZQV/W/Vf BY fizww/ AV70E/VFKK Oct. 3, 967 E. v. JENNINGS 3 7 POWDER EVAPORATION APPARATUS Filed Aug. 30, 1965 2 Sheets-Sheet 2 United States Patent O 3,344,768 POWDER EVAPORATTON APPARATUS Ermont V. Jennings, Covina, Calif., assignor to Burroughs Corporation, Detroit, Mich, a corporation of Michigan Filed Aug. 30, 1965, Ser. No. 483,570 13 Claims. (Cl. 118-48) This invention relates in general to sublimating apparatus and more particularly relates to a new and improved radiated container for the evaporation of powder.

Thin layers of evaporated powder are necessary in numerous and diverse areas of electronic equipment. The technique of evaporating a powdered chemical under controlled conditions has proved the most successful thin layer depositing technique. In general, this technique requires a heat source which is normally provided by a current conducting filament. This filament is normally placed in direct contact with the powdered chemical that is to be evaporated, and in prior art approaches, has taken many different forms and shapes.

Almost all powders which are utilized for evaporation techniques today are extremely poor heat conductors. This poor heat conduction of powder in direct contact with a filament causes problems of powder evaporation which, prior to this invention, had not been satisfactorily solved by the prior art. One result is that the powder does not heat evenly but jumps and spits as does popcorn on a hot pan. Because of this spitting and jumping, solid hunks of powder are often propelled with considerable force against the surface to be plated, and these hunks produce pinholes in the film that is being evaporated on the surface. Attempts have been made to combat the explosion of small powder particles in the past, and these attempts are discussed hereinafter following a brief discussion of other adverse effects from the poor conductivity of powders to be evaporated. Direct contact of powder with a hot surface causes the powder to sublimate directly to a vapor state thus creating a space between the heat source and the remaining non-evaporated powder. A crust often develops at these regions of non-evaporated powder spaced away from the heat source, and this crust shields the powder from any further evaporation. The residue thus produced represents Wasted powder material. This drawback further prevents utilization of known techniques for computing the thickness layer of deposited film, because it is difficult if not impossible to accurately measure the content of powder evaporated.

'Prior art attempts to solve the foregoing problems, in general, have failed. Such attempts have included completely embedding a current conducting wire in a thick powder cube. The direct heat from the current in the filament causes the powder in contact with the wire to boil away and evaporate through a filter which is in this case the outer areas of the thick powder cube itself. This technique does not completely eliminate the particle throwing mentioned above in that cracks in the powder often develop, and such cracks provide a direct path for powder particles to strike the substrate surface to be coated.

Furthermore, the entire film structure, for most electrical uses, must have a consistent characteristic which is not provided by the prior art. The direct heat boils the powder in contact with the filament heater at a high evaporating temperature, which temperature varies over a wide range as contrasted with the temperature of evaporation of the powder further away from the filament. These varying evaporation temperatures create different film characteristics at different evaporation temperatures, and thus films coated in this manner are totally unacceptable for some uses. Other techniques such as compressed powder disks, various shaped crucibles, and

battles and shields for developing a tortuous path, represent in varying degrees, at least one or more of the foregoing disadvantages of this prior art. In summary, the plating techniques of the prior art have failed to provide a thin fihn of controlled characteristics, density, thickness, uniformity and stress.

The foregoing disadvantages of the prior art are avoided by the principles and features of this invention wherein uniform radiant heating completely evaporates a powder to be evaporated without resulting in pinholes in the surface to be coated with powder vapor. A new and improved container is provided in association with a radiation furnace, or heat source, having an aperture at the top, sealed at the bottom and shielded to radiate heat inward. This source is utilized to provide essentially an even temperature in the heating area wherein a powder container of a new and improved structure is placed. This container, in one embodiment, comprises two hollow impervious cones joined at their bases -by a hollow cylinder formed from a tightly wound wire spiral. The bottom cone is loaded with a known amount of powder and is supported in the heater area by fingers attached thereto and supported at the opening in the furnace structure. The cylindrical wire between the top and bottom cones exposes minute openings between overlays in the tightly wound wire, which openings allow escape of only evaporated powder. The conical shaped top and bottom provide an even surface temperature such that a known quantity of powdered material is completely evaporated at a known and constant temperature of evaporation. Powder particles cannot pass through the cylindrical wire nor through the impervious cones and any that may escape through the wire are propelled directly against the furnace heater. At this heater such particles are instantly vaporized, and therefore are prevented from reaching the substrate to be coated. Thus a pinhole-free film of evaporated powder is of uniform thickness and structure, and consistent characteristic throughout is easily and readily obtained by the principles and features of this invention.

The foregoing principles and features of this invention may more readily be understood by reference to the accompanying drawing in which:

FIG. 1 is a perspective view in exploded form of a container and furnace in accordance with the principles of this invention; and

FIG. 2 is a side elevation of a powder evaporating unit including the components of FIG. 1 in assembled form.

Turning now to the drawing in FIG. 1, a container 8 for a powder to be evaporated and a furnace for supplying radiant heat during the evaporation process is depicted. The container 8 comprises a first hollow-bottom cone 10 and one hollow cylindrical section of tightly wound wire 11. A top cone 12 with another cylindrical section 13 is provided for joining with the first cylinder 11 in order to establish a closed container when fully assembled.

The material for the cones 10 and 12, and the spiral wound cylinder sections 11 and 13 may advantageously be an easily machinable pure material such as tantalum. Other pure high temperature resistant materials, however, may be used equally well. Such other materials include tungsten, niobium and molybdenum, to name only some possible alternatives.

The top and bottom halves, each including a cone and a cylinder, of container 8 are provided with reinforcing ribs 6 that mesh with their opposed cylinders as the container is put together, in order to add increased stability and a tighter fitting joint between the two halves. The bottom half of the container 8 further includes three supporting fingers 5 which are welded or otherwise fastened to cone and cylinder 11. These fingers Shave a transverse right-angle bend at their uppermost ends to provide an extension which rests on the lip of opening 14 in furnace 15 when the container is assembled and put into operation, as shown in FIG. 2 and described hereinafter. These fingers 5 serve to position the container 8 at approximately the midpoint of the opening in furnace 15, and further centers the container 8 with its sides a small separated distance from the hot surface areas of furnace 15.

A side elevation of the container and furnace assembly is depicted in FIG. 2 which illustrates one possible standard approach for utilizing such equipment in thin film evaporating art. A substrate material 20, which may be any platable material such as glass, metal, etc., is supportably positioned in an evacuated chamber 19 by support arm 21. Evacuation of chamber 19 may also be accomplished in any well-known manner. For example, a vacuum bell 22 may be bolted or otherwise fastened to a base plate 23 in order to form an airtight seal by the use of sealing rings 24 in a manner standard in the art. Any suitable vacuum pump 25 and associated valve 26 may be employed to reduce substantially all oxygen and other gas ions from the area 19 within vacuum bell 22. In this regard, thin film depositing is accomplished at different vacuum pressures depending upon the precision and consistency necessary for the film layer on the substrate surface 20.

In the following discussion the container material is assumed to be tantalum metal as is the furnace material. This metal is chosen because of its high temperature resistance and its easy machinabiilty. The powder to be evaporated is assumed to be silicon monoxide. It should be understood, however, that this material and this powder are merely examples, and numerous other materials are suitable for use. Some such other materials for the furnace and container have been listed hereinbefore; and, of course other powders may be deposited by this thin film technique.

Furnace 15 may be any well-known furnace of the inward radiation type having a radiation shield 16 placed between the outer Wall 17 and the inner wall 18. Furnace 15 includes an upper and lower disk 31 and 32 respectively which are attached to two posts 33 and 34. These posts are current conducting busses, typical in vacuum bell jars, and are insulated by airtight seals at base 23 in the manner shown. A voltage source 36 connected to the downwardly extending posts 33 and 34, supplies current to radiation furnace 15. Because of the extremely low resistance of the tantalum metal a small applied voltage is chosen at source 36 which creates a high current flowing through the inner wall 18 which is the hot furnace chimney. This high current heats chimney 18 of the radiation furnace 15 to the required evaporation, or sublimation temperature, of the powder to be evaporated. Radiation shields 16 and 17 are suitably spaced to maintain the inward area of chimney 18 a substantially even temperature, to within less than a 50 centrigrade gradient.

The new and improved container of this invention is held within this uniformly heated area of chimney 18 by supporting fingers 5. The surface area of container 8 takes advantage of this low temperature gradient by pre senting surface area shaped to conduct the radiant heat received thereat uniformly throughout the powder to be evaporated as described hereinafter.

A power 35 to be evaporated, such as silicon monoxide, is deposited in the bottom half 10, 11 of con tainer 8. The container is closed by seating its upper half 12, 13 in place in the manner shown in FIG. 2 so that the cylindrical wire sections 11 and 13 form a cylindrical wall of wire overlaid with spacing for vapor to escape from within the container 8. Silicon monoxide power 35 sublimates at high temperatures such as that produced by radiation furnace 15 and upon sublimation power 35 I of these arrows 38 should be understood as emanating from an equivalent surface source and therefore follows the familiar cosine law of deposition.

By choosing the angle of the bottom cone 10 of container 8 at approximately 45 to 75 from the horizontal I have found experimentally that the radiant heat is distributed evenly throughout the powder 35 with the result that it sublimates completely and after evaporation there is no residue remaining in container 8. Any tendency to spit or pop which may occassionally be present does not produce any adverse effects on substrate surface 20 because upper cone 12 is impervious to the escape of any powder particles whatsoever. The tightly wound cylinder areas 11 and 13 prevent the escape of any but the smallest sized powder particles. Any particles which do manage to escape the overlaid wire, however, do so substantially horizontally. This angle of departure for stray powder particles assures immediate contact with chimney 18 of radiation furnace 15. Chimney 18, of course, is the current conducting region of furnace 15 and is extremely hot with the result that any occasional powder particles coming through the spiral winding areas 11 and 13 are instantly vaporized.

This conical section 10 may be the same angle as that chosen for the bottom conical section 12, however, I have found that an additional advantage is gained by choosing the larger top conical angle approximately 60 measured from the horizontal at the base of cone 12, because at this angle each point of the conical surface is looking at or exposed to the hot chimney 18 of radiation furnace I15. It is obvious, of course, that the very apex of the top cone 10 is directed toward the vacuum and thus is not radially exposed to a furnace area. However, by choosing the angle of cone 12 larger than that for cone 10, and makingthe upper cone slightly larger by this procedure, every point along the cone 10 with the exception of the very apex, experiments have shown, is substantially at the same temperature as the remaining areas of container 8, Tantalum metal, of course, does conduct heat and thus the apex is heated by conduction from the lower areas of cone 12, which lower areas are directly exposed to radiant heat.

It should be understood in the specification and appended claims that the cylindrical sections 11 and 13 of tightly wound wire are merely representative of one shape of cylinder. Numerous other cylinder shapes, and top and bottom esctions 1t) and 12 as well, may be devised which employ the principles of this invention. For example, the tightly wound wire may form a hollow rectangular column having two hollow pyramids at each end. Obviously, some shapes for the powder container will require a correspondingly shaped radiation heat source so as to assure an even distribution of the heat through the container. This even distribution of radiant heat, provided by upper and lower caps with a vertex and a base to match the middle cylinder, will thus provide complete evaporation of the powder charge.

An initial charge of powder 35, which is completely evaporated by this invention, can be measured beforehand, with precision. It is well known that complete evaporation of a known quantity of charge at a known evaporation temperature and vacuum, provides a basis for determin ing the thickness measurement of the thin film deposited on substrate 21 In prior art containers and furnace assemblies the presence of residue in the container has prevented this known technique of measuring the thickness of the thin film and has forced instead the employment of complicated and expensive measuring devices for obtaining film thickness.

A further advantage gained by the container of this invention is that the container is constructed of tantalum Wire and sheets which are substantially thicker than any prior art containers, and thus the life of the container is prolonged and is reuseable over numerous evaporation cycles. Different thicknesses of tantalum material can be employed. However, for the container 8 of this inven tion the conical sections may be made from a tantalum sheet .002 inch thick, and the cylindrical wire for the spiral cylinder section 11 and I13 may be chosen of wire .02 inch in diameter. With the tantalum wire tightly overlaid or convoluted, gaps of less than .001 of an inch are present between adjacent convolutions of the tantalum wire. These gaps allow escape only of evaporated powder, or powder particles so small and at such an angle of escape that they are positively evaporated upon striking the hot inner shield surface area 18.

It is to be understood that the foregoing features and principles of this invention are merely descriptive, and that many departures and variations thereof are possible by those skilled in the art, without departing from the spirit and scope of this invention.

What is claimed is:

1. A powder evaporation apparatus comprising a closed chamber for holding powder to be evaporated, said chamber comprising a hollow cylinder formed from a tightly wound wire convolute to form spaces between convolutions for escape only of vapor from within said container, at first impervious section having a vertex and a base matched to and fastened at one end of the cylinder and a second similar shaped impervious section matched to and fastened at the other end of the cylinder, and a source of radiant heat for evenly distributing heat throughout all portions of said chamber whereby the contained powder is substantially completely evaporated.

2. A powder evaporation apparatus in accordance with claim 1 wherein said source of radiant heat includes an opening and further comprising a substrate surface positioned near said opening for receiving a uniform layer of evaporated powder deposited on said surface.

3. A powder evaporation apparatus in accordance with claim 2 wherein said first and second sections are conical shaped.

4. A powder evaporation apparatus in accordance with claim 3 wherein the vertex of said first conical shaped section is positioned near said opening in said source of radiant heat.

5. A powder evaporation apparatus in accordance with claim 4 wherein the angle defining the vertex of said first conical section is smaller than the angle defining the vertex of the second conical section remote from the opening in said heat source for providing substantially uniform heat within the closed container.

6. A powder evaporation apparatus comprising a closed chamber for holding powder to be evaporated, said chamber comprising a hollow cylinder formed from a tightly wound wire overlaid to form spaces between layers for escape only of vapor from within said container, a first impervious section having a vertex and a base matched to and fastened at one end of the cylinder and a second similar shaped impervious section matched to and fastened at the other end of the cylinder, the vertex angle of said first section being smaller than the vertex angle of said second section for evenly distributing heat throughout all portions of said container, a source of radiant heat surrounding and supplying heat for said chamber and having an opening centered above the vertex of the first section, and a substrate surface to be deposited by the substantially complete sublimation of powder to vapor positioned above said opening.

7. A powder evaporation apparatus in accordance with claim 6 wherein said cylinder is separable for allowing the introduction of a measured charge of powder, and further comprising means attached to said cylinder for securing both parts thereof when reassembled.

8. A vapor deposition apparatus comprising a vacuum chamber, a furnace of radiant heat in said chamber, and a container for powder in said furnace, said container including successive convolutions of a closely wound wire refractory metallic material forming a cylinder having an impervious cone at each end with the base of each cone fastened to an end of the cylinder.

9. A vapor deposition apparatus in accordance with claim 8 wherein said metallic material for said cylinder is of tantalum wire and wherein said cones are formed from solid tantalum sheets.

10. A vapor deposition apparatus in accordance with claim 9 wherein said powder container is substantially surrounded by said furnace with an apex of one cone pointing to an opening in said furnace.

11. A vapor deposition apparatus in accordance with claim 10 wherein said container is suspended with said apex near said opening by upwardly extending support means fastened to said container and projecting into and away from said opening.

12. A uniform film depositing apparatus comprising an evacuated chamber, a surface to be coated with a material, a powdered chemical of the material to be coated having a sublimation temperature, a container for said powdered chemical, said container including opposed similarly shaped impervious sections each having a vertex and a base attached to a center section of tightly wound overlaid wire forming spaces between wire layers for escape only of vapor from within said container, a furnace surrounding said container and having an opening positioned between one vertex of the container and said surface to be coated, and electric supply means connected to said furnace for developing radiated heat uniformly within said container of at least said sublimation temperature.

13. A uniform film depositing apparatus in accordance with claim 12 wherein said center section of said container is divisible in two parts, one each of which is attached to the base of the opposed impervious sections, for receiving a charge of powder.

References Cited UNITED STATES PATENTS 611,047 9/1898 Wheeler 11848 X 3,092,511 6/1963 Edelman l17107.2 3,129,315 4/1964 Rudke et al 11849 X 3,153,137 10/1964 Drumheller 2l9271 3,244,857 4/1966 Bertelsen et al. 219275 MORRIS KAPLAN, Primary Examiner. 

8. A VAPOR DEPOSITION APPARATUS COMPRISING A VACUUM CHAMBER, A FURNACE OF RADIANT HEAT IN SAID CHAMBER, AND A CONTAINER FOR POWDER IN SAID FURNACE, SAID CONTAINER INCLUDING SUCCESSIVE CONVOLUTIONS OF A CLOSELY WOUND WIRE REFRACTORY METALLIC MATERIAL FORMING A CYLINDER HAVING AN IMPREVIOUS CONE AT EACH END WITH THE BASE OF EACH CONE FASTENED TO AN END OF THE CYLINDER. 